1. Field of the Invention
The present invention relates to a beam shaping prism designed to create an orthogonal optical path and an optical pickup employing the same.
2. Description of the Related Art
As interest in mobile recording devices increases, demands for optical recording applications using low-priced media increase and optical pickups for mobile applications are being actively developed.
An optical pickup for mobile applications requires high output power of a laser diode (LD) used as a light source for recording, which leads to high power consumption and heating value.
To overcome these problems, one solution is to increase the efficiency of light utilization by shaping a laser beam with a beam shaping prism, thereby realizing a system for recording with lower output power requirements. A beam shaping prism is commonly used in an optical pickup for recording due to the profile of an output beam from a LD.
FIG. 1 is a schematic diagram of a typical LD that emits a laser beam from a side. Referring to FIG. 1. a laser beam having an elliptical cross-section is emitted from an activation layer 3 of a LD 1. The elliptical beam profile is due to a difference between thickness (vertical dimension) and width (horizontal dimension) of the activation layer 3.
Due to the geometry and structure of the activation layer 3, vertical and horizontal diffraction angles differ so angles θ∥ and θ⊥, which the beam emitted from the LD 1 diverges in horizontal and vertical directions, respectively, differ from each other. Thus, the beam diverging in the horizontal direction appears to have been emitted from a location that is a distance ΔZ behind a location from which the beam diverges in vertical direction. The distance difference ΔZ between starting points where the beam diverges in the vertical and horizontal directions is called an “astigmatic difference”.
In this case, the main polarization direction of a beam emitted by the LD 1 is a minor axis (i.e., horizontal) direction of the beam. Thus, due to the astigmatic difference, the LD 1 commonly used as a light source in an optical pickup emits an elliptical beam polarized in the minor axis direction.
In contrast to the LD 1 emitting an elliptical beam, an objective lens with a circular aperture needs a circular beam in order to increase the efficiency of light utilization. Thus, to achieve the same purpose, an optical pickup for recording commonly uses a beam shaping prism to convert an elliptical beam emitted by the LD 1 into a circular beam.
FIG. 2 shows an example of a conventional optical pickup. Referring to FIG. 2, the conventional optical pickup includes a LD 11, a beam shaping prism 20 that shapes an elliptical beam emitted by the LD 11 into a circular beam and changes the propagation path of the beam, an objective lens 19 that focuses an incident beam to a spot on an optical disc 10, and a photodetector 29 that receives a beam reflected from the optical disc 10.
A collimating lens 13 collimates the beam emitted from the laser diode 11 to a parallel beam that then passes through a grating 15 and is incident onto the beam shaping prism 20. The beam shaping prism 20 is comprised of first and second prisms 21 and 23 that shape the incident elliptical beam into a circular beam and a third prism 25 that reflects a beam reflected off a mirror surface between the first and second prisms 21 and 23.
The beam passing through the first and second prisms 21 and 23 is shaped into a circular beam that is then reflected from a reflecting mirror 17 into the objective lens 19. The beam is focused onto the optical disc 10 by the objective lens 19. The beam reflected from the optical disc passes through the objective lens 19 and the reflecting mirror 17, is incident onto the second prism 23, and is reflected from the mirror surface between the first and second prisms 21 and 23 into the third prism 25. The beam incident on the third prism 25 is internally reflected from an inclined surface toward the photodetector 29, is changed into a convergent beam through a collimating lens 27, and is focused onto the photodetector 29 through a sensing lens 28.
In the conventional optical pickup, an optical axis of a beam emitted from the LD 11 onto the beam shaping prism 20 is not perpendicular to an optical axis of a beam emerging from the beam shaping prism 20 toward the objective lens 19. Thus, optical elements are arranged at designated angles to each other.
Fixing components at designated angles to each other deteriorates precision in manufacturing a base on which all the optical pickup system elements except an objective lens are arranged and assembled, thus causing difficulties in precisely assembling the elements. That is, it is difficult to control tolerances during the manufacture of the base and assembly and alignment of the elements, as opposed to when the two optical axes are orthogonal to or coincident with each other.